1. Field of the Invention
The present invention relates primarily to an automatic steering device. More precisely, it relates to a configuration for better stabilizing the course in an automatic steering device.
2. Background Information
It generally takes a certain amount of time for the hull of a ship to start turning after a steering input. And once the hull begins to turn, that turning will continue for a while after the rudder has been returned to its neutral position, so the rudder must be returned quickly in order to assume the desired course. The steering of a ship thus entails its own unique difficulty, and considerable skill is required to reliably and quickly carry out the operation needed to bring the prow of a ship around to the desired orientation. Also, at sea a ship is affected by waves, wind, and other such external disturbances, so steering that will cancel out these external disturbances is required in order to keep the vessel proceeding in a straight line. Because of the above-mentioned steering difficulty, however, just keeping the vessel going straight demands considerable concentration, and this imposes a burden on the helmsman.
A maritime automatic steering device (auto pilot) is installed in vessels to lessen this burden on the helmsman. A typical automatic steering device is configured such that the heading in which the prow is supposed to be pointed (the charted course) is inputted as a target value, and the steering gear (an operation element) is operated to automatically change the rudder angle, thereby matching the heading (a control amount) with the charted course. This control can be performed by a known method. Well known examples of such methods include acquiring a deviation angle of the heading with respect to the target heading and turning the rudder in proportion to this deviation angle (proportional control; P control), turning the rudder in proportion to the sum of past deviation angles (integral control; I control), and acquiring the rate of change of the deviation angle and turning the rudder in proportion to this rate of change (differential control; D control), among others.
Automatic steering devices of this type are disclosed in Japanese Unexamined Patent Application Publication Nos. S62-113695 (Patent Literature 1) and S54-33493 (Patent Literature 2), for example. With the maritime automatic steering device in Patent Literature 1, an estimated heading signal and an estimated turning angular velocity signal are found from a heading signal outputted by a gyro compass, an optimal rudder angle command is calculated based on this, and this command is outputted to the steering gear. Patent Literature 2 discloses an online, real-time, optimal steering system that is incorporated into an auto pilot stand, which is configured such that external disturbances in the ocean or the sea region in which the vessel is sailing are measured and analyzed during operation, and an evaluation function for optimal control is varied according to these external disturbances.